When oil is produced from unconsolidated or poorly consolidated formations, migration into the wellbore of loose sand and erosion from sandstone is a constant problem. This migration of sand may eventually clog flow passages in the production system of the well and can erode downhole and surface equipment. In some instances, the clogging of the production system may lead to complete cessation of flow, or "killing" of the well.
Sand migration is typically controlled by placement of a gravel pack around a slotted liner or a wire-wrapped screen. For simplicity, the slotted liner or wire-wrapped screen will be referred to herein as a "liner". The "gravel" used in such a gravel pack is typically a sand of a size which is large enough to be kept out of the production liner or screen, but small enough to prevent migration of formation sand past the gravel pack and into the production facilities.
A gravel pack is generally placed by first inserting the liner in the portion of the wellbore to be packed. The wellbore may be either cased or uncased. The wellbore is generally cased in poorly consolidated or unconsolidated formations. A "crossover" tool is positioned on top of the liner. The crossover tool and liner are suspended from a workstring. The crossover tool allows a slurry of sand in a carrier fluid to be pumped down a workstring to the crossover tool, and then be routed through the crossover tool to the volume surrounding the liner. The sand is deposited both within this volume and the perforations. In a circulating gravel pack, some of the liquid carrier of the gravel pack slurry enters the liner where it can communicate through on to the annulus surrounding the workstring and be returned to the surface. The rest of the fluid leaks out to the formation through the perforations. The percentage of the carrier fluid which returns to the surface is referred to as the return rate. A packer is placed above the liner in the annulus surrounding the crossover tool to prevent the slurry around the liner from bypassing the liner and communicating directly to the annulus around the workstring. After the sand is deposited around the liner and in the perforations, the crossover tool is disconnected from the liner and lifted out of the wellbore. A production tubing is then lowered into the wellbore and connected to the liner.
A slurry which is relatively viscous will carry sand into perforations effectively, but will not result in a good gravel packing in the annulus surrounding the liner. A less viscous carrier fluid such as water without a thickener is effective in packing the annulus around the liner, but is not effective to carry gravel into perforations. J. K. Forrest, in "Horizontal Gravel Packing Studies in a Full-Scale Model Wellbore," SPE paper 20681, and in U.S. Pat. No. 5,058,677, suggests a two stage gravel packing process to accomplish packing of both perforations and the wellbore. A hydroxyethyl cellulose (HEC) containing slurry is used to pack the perforations and about 75% of the liner-casing annulus. A slurry that does not contain a thickener is then used to complete the gravel pack. For best results, the HEC containing slurry was followed by a "HEC post pad." This process therefore requires many steps resulting in a complicated and time consuming gravel packing process.
The use of thickeners such as HEC in the concentrations known in the prior art requires a breaker to reduce the viscosity of the slurry and allow settlement of sand after the slurry is placed in the wellbore. Most breakers reduce the viscosity of the slurry at a rate that is strongly dependent upon temperature and the amount of shear to which the slurry is exposed. It is difficult to control timing of the preparation of the breaker containing slurry, injection into the wellbore and slurry temperature. A relatively slowly acting breaker is therefore used to ensure that the breaker does not reduce the viscosity of the slurry too quickly, resulting in the sand prematurely settling from the slurry, or "sanding out" prior to placement in the wellbore and perforations. It would be preferable to utilize slurries that do not require the use of breakers.
A useful polysaccharide for imparting viscosity to gravel pack slurries is succinoglycan. This polymer and its properties are disclosed in, for example, ACS Symposium Series 396 "Oil Field Chemistry--Enhanced Recovery and Production Simulation" by A. J. Clarke-Sturman, et al. Chapter 8, pp 157-168; "Succinoglycan: A New Biopolymer for the Oil Field" by A. J. Clarke-Sturman, et al. from Symposium on Advanced Oil Field Chemistry presented before the Division of Petroleum Chemistry, Inc. ACS, Toronto Meeting, Jun. 5-11, 1988; and European Patent Application 0,040,445. These polymers impart a high and relatively temperature insensitive viscosity to an aqueous composition below the polymers, transition temperature. As the transition temperature is exceeded, the viscosity imparted to an aqueous mixture by these polymers decreases markedly. The transition temperatures may be adjusted to a certain extent by addition of various modifiers, as disclosed in Great Britain Patent Application No. 902 6596.8 and the A. J. Clark-Sturman articles cited above.
It is therefore an object of the present invention to provide a process to gravel pack a wellbore that is effective at packing both the wellbore and perforations. It is also an object to provide such a process that is accomplished using the same gravel pack slurry to pack both the perforations and the wellbore. In another aspect it is an object of the present invention to provide a slurry capable of packing both the perforations and the wellbore. It is also an object to provide such a process and such a slurry that are effective to pack perforated deviated wellbores.